Free-Operant Discounting: Initial Results From an Adjusting-Duration Procedure

Rats responded under a procedure in which relatively short lever-press durations produced 1 food pellet and longer presses produced 4 food pellets. Because duration is a continuous dimension of behavior, it was possible to arrange conditions in which a smaller reinforcer was available up until the moment that criterion for earning a larger reinforcer was satisfied. The difference in reinforcement criteria for the 2 reinforcer options adjusted during the session based on the rats' performance. Although strong conclusions were limited by a lack of stability in some phases, response durations appeared to be sensitive to both reinforcer magnitude and duration criteria. Median durations decreased for all rats when 4-pellet deliveries were no longer available, and they increased when 4-pellet deliveries were reintroduced. In addition, median durations decreased for all rats exposed to probe sessions in which 4 pellets were available immediately after lever depression. We propose that this assay provides a measure of impulsivity that may complement existing delay-discounting procedures.